Laboratory Tests: Blood Gases, Anion Gap, and Strong Ion Gap

Oxygen Tension and Hemoglobin Saturation

Oxygen tension, or the partial pressure of oxygen (PO ₂ ), is the activity of the molecules of oxygen dissolved in the plasma. It is measured with a Clark electrode at a temperature of 37°C. Hemoglobin oxygen saturation (SatHb or SO ₂ ), measured by infrared spectroscopy, is the fraction of oxygen bound to hemoglobin molecules relative to the total amount that could be bound. The relationship between the oxygen tension and hemoglobin saturation is represented by the O ₂ -Hb dissociation curve. The affinity between O ₂ and Hb is reduced with increases in 2,3-diphosphoglyceric acid (DPG), CO ₂ , and temperature. However, the major regulator of O ₂ affinity is plasma pH. Shifts in oxyhemoglobin dissociation are easily understood from a teleological perspective: increased tissue metabolism leads to local increases in DPG, CO ₂ , temperature and/or H ⁺ , which cause increased oxygen offloading from hemoglobin. The situation is reversed during alveolar transit in the lungs, increasing Hb-O ₂ binding and facilitating oxygen uptake.

By sampling from an arterial and (mixed or central) venous source, the oxygen extraction ratio can be calculated and used to assess tissue metabolism, global hemodynamics, and the clinical significance of anemia.

The total arterial blood oxygen content (CaO ₂ ) is determined by the hemoglobin concentration, the hemoglobin saturation, and the freely dissolved O ₂ as a function of PaO ₂ . It can be approximated by the following formula:

Carbon Dioxide Tension

The partial pressure of carbon dioxide (PCO ₂ ) measures the activity of the carbon dioxide molecules dissolved in plasma. The PCO ₂ is measured by the Stow-Severinghaus electrode and normal values of arterial PCO ₂ range from 35 to 45 mm Hg. The solubility coefficient is 0.0306 mmoL/mmHg/L, so at a PaCO ₂ of 40 mm Hg, the CO ₂ molecules dissolved into the plasma amount to 1.224 mmol/L, corresponding to 27.4 mL of CO ₂ per dL of plasma.